The present invention relates to an apparatus for the quality control of a print produced by a printing machine. The apparatus comprises a CCD camera and a measuring girder having a length which essentially covers the width of a printing surface to be controlled.
A problem in the printing industry and especially in newspaper printing is the quality control of printed matter from a high-speed web of paper. In newspaper production, a paper emerges as a finished product from a printing machine and, thus, it cannot be checked afterwards except by picking up the product and glancing through it.
Quality problems result from malfunctions occurring in the watering or inking device of an offset printing machine, such as accumulation of paper dust in pipe systems, mechanical and electrical malfunctions or just inadequate maintenance. A reader experiences the quality problems as poor quality printing either in part of a page or in several pages. The problem becomes even more pronounced if a poor quality page carries a sizable advertisement or several advertisements since advertisments represent a major source of income for a newspaper.
In so-called roll-to-roll printing machines, the printing industry has earlier employed a web inspection device which was based on a rotating mirror polygon which, upon rotating at a correct speed, brought the web to a halt for inspection. Such a device required an operating person and was therefore only used for occasional inspection. At present, this application has been replaced by a CCD camera, which snatches an image of a web of paper onto a colour monitor to be inspected by a printer. There are also measuring girders carrying a plurality of CCD cameras side by side and measuring girders effecting the measurement by means of separate optical transducers.
These products as such are good but expensive. A newspaper printing machine usually includes one to six webs of paper, whereby the number of such measuring devices should be twice that of the webs as a newspaper page is printed on both sides. The existing products are difficult to apply for measuring a wide web of paper as there will be a long distance between the camera and the object to be measured. In a heavy-duty printing machine there is very little space available and hence, in addition to being inexpensive, the product must also be sufficiently small. The environmental aspects of a printing machine are also demanding: the conditions are humid (even wet) and temperature may rise up to 30.degree. C.